The invention relates to a device for centering and clamping a flexible foil with a round opening, for example, a round magnetic record carrier, on a bell-shaped hub which rotates the foil in the plane of its major surface, which hub has a cylindrical inner bore whose diameter is slightly greater than the diameter of the opening in the foil, and has an end surface on which the foil is positioned, the opening in the foil permitting the insertion of an expandable expanding mandril having a conical portion and a cylindrical portion into the inner bore of the hub, thereby keeping the foil positioned on the end surface of the hub with a flange having a contact surface which is parallel to said end surface. After the foil has been positioned on the end surface of the hub, an expanding cone, which can be inserted into the expands mandril, expanding the mandril so far that the cylindrical portion of the mandril is urged against the wall of the cylindrical inner bore of the hub, thereby enlarging the opening in the foil to the diameter of the inner bore.
Such a device is known from DE-AS No. 23 30 818. In said device, the expanding mandril has a conical inner surface. The generating angle of said conical inner surface is smaller than the generating angle of the circumferential surface of the expanding cone. When the expanding cone is inserted, the expanding mandril is therefore initially expanded until its cylindrical portion engages with the inner bore of the hub and the foil opening is enlarged to the diameter of said bore. As the expanding cone is inserted further, the expanding mandril is urged further into the inner bore of the hub so that the flange of the expanding mandril presses the foil onto the end surface of the hub.
In order to impart a torque to the foil, a specific minimum pressure of the flange of the expanding mandril is necessary, which, in the ideal case, should be directed parallel to the hub axis and which should at least be transmitted to the expanding mandril by the expanding cone. However, since the contact surface between the expanding mandril and the expanding cone is inclined relative to their axis, the transmission of force from the expanding cone to the expanding mandril also gives rise to a radial component which causes the expanding mandril to widen underneath the flange and depends on the inclination of the contact surface relative to the axis and the clamping force required for the foil. This radial force component gives rise to an additional expansion of the expanding mandril underneath the flange; the upper portion of the expanding mandril near the flange being tilted in an inward direction towards the axis owing to the necessarily rigid wall of the expanding mandril, thereby pulling the flexible foil inwards. This results both in centering errors and in uncontrolled bulging of the flexible foil. The centering error gives rise to an inaccurate position of the magnetic head relative to the desired track and thus to a substantial reduction of the read signal and the uncontrolled bulging of the flexible foil leads to at least a temporary loss of contact of the magnetic head with the foil.